1. Field of the Invention
The present invention is concerned with ultrasonic testing generally, and more particularly with echo-type ultrasonic testing, sometimes referred to as ultrasonic scanning echography, in which a transducer is utilized to generate a beam of ultrasonic sound which is reflected back to the transducer from a workpiece being inspected. The reflected ultrasonic sound waves are displayed to detect anomalies in the workpiece. Such techniques are extensively used for non-destructive testing of welded or solid pieces in which structural anomalies such as cracks, pinholes, discontinuities and the like cause characterisic sound wave echoes which indicate the existence and position of such anomalies.
2. Description of Related Art
Perdijon U.S. Pat. No. 4,557,145 discloses an apparatus for ultrasonic testing of workpieces in which the transducer is rotated to define a conical surface about an axis which is orthogonal to the inspection surface of the workpiece, as illustrated in FIG. 1 of the patent. The rotating transducer is translated across the surface of the workpiece and the patentee states that the ultrasonic sound pulses are timed so that each zone to be scanned receives at least two pulses corresponding to different azimuth angles about a common axis of rotation.
Garner et al U.S. Pat. No. 4,685,966 shows an apparatus which enables compound movement of the ultrasonic probe in order to maintain the angle of incidence at or close to perpendicular to the surface of the workpiece, even when the latter is a compound curved surface. The patent discloses rotation about both a horizontal and vertical axis (e.g., column 2, line 45 through column 3, line 19) with the objective of always maintaining the angle of incidence substantially perpendicular to the test surface.
Spencer et al U.S. Pat. No. 3,765,229 discloses an ultrasonic scanner which is interchangeable between an oscillating mode to examine curved panels as illustrated in FIG. 4, and a translational mode for the examination of flat panels. See the paragraph headed "Operation" in column 4 of the patent.
Singh et al U.S. Pat. No. 4,502,331 shows ultrasonic inspection of turbine disc rims in which, as illustrated in FIG. 6, the transducer is skewed in different passes at 45 and 60 degrees in order to inspect, respectively, the concave and convex sides of the "steeples" of the workpiece.
Conventional ultrasonic echo pulse testing, sometimes referred to as "echography", generally examines a workpiece by scanning a surface or region of the workpiece, often by carrying out two mutually perpendicular series of passes along the same surface or region. Thus, a rectangular region may be scanned from top to bottom in a first series of parallel, north-to-south passes and from side to side in a second series of parallel east-to-west passes. In the case of a disc-shaped workpiece, the scanning may be conducted along one or both of its circular-shaped surfaces and along its cylindrical-shaped surface. Conventional respective orientations of the test probe relative to the workpiece in respective series of passes are sometimes referred to as axial, radial and circumferential orientations, as described in more detail below. The orientations employed may be mandated by the direction of metal grain flow in forged articles, and by the desire to simplify the test set-up. This conventional approach accepts the uninspectability of certain regions of a workpiece because of reflection of the ultrasonic sound pulses from lands, bosses, ridges, grooves, rims or other return-reflecting formations or configurations of the workpiece. Such reflections, i.e., echoes, mask the much smaller echoes caused by anomalies in the workpiece. Even in the case of a simple shape, such as a flat, rectangular plate, the edges of the workpiece which are transverse to the direction of a given scan or series of scans are uninspectable because of ultrasound reflection to the probe from the edges. As a consequence, regions of the workpiece are either left uninspected, resulting in inherent risks and lower reliability of the tested part, or must be tested by other techniques such as X-ray, metal filings dispersion, and the like. The requirement for additional tests, usually carried out with different equipment, naturally increases substantially the test cost per workpiece. Even if the additional testing to inspect areas ininspectable by conventional orientation of the test probe is carried out with ultrasonic echo pulse testing on the same equipment, it requires a third and possibly additional series of inspection passes, as well as the calculation of probe positioning for additional set-ups, in order to scan as much as possible of the workpiece volume which is uninspectable with the conventional axial and circumferential probe orientation.